Power converters (for instance, DC/DC power converters) are used to provide alternate levels of DC voltage from a primary source of DC voltage. These converters customarily use switching devices that convert the DC input voltage into an AC voltage to drive a primary winding of a transformer thereby allowing the voltages in the secondary side of the transformer to be selected to meet the load requirements. The switching devices are usually operated at relatively high switching frequencies to allow the use of smaller components such as inductors and capacitors within the converter. As a result, the parasitic or stray inductances or capacitances associated with the components of the converter can be reduced.
The parasitic elements mentioned above, however, generate high frequency oscillations that appear as undesired "ringing" waveforms in the converter. The ringing waveforms prompt the use of higher rated and higher cost circuit components in order to operate in such an environment. Additionally, the deleterious ringing causes the converter to be lossy and less efficient. Some of the loss manifests itself as undesirable electromagnetic interference (EMI) causing regulatory problems which must be addressed. Due to the relatively small resistance values of the transformer and inductor elements, the ringing energy may only be lightly damped in the converter.
The spurious ringing necessitates that a rectifier (e.g., diodes) with higher reverse voltage ratings be employed in the converter. For example, if the ordinary reverse voltage in the converter is 200 volts and the added ringing voltage generates 400 volts, the rectifier diode must conservatively have a reverse voltage rating of about 600 volts. A 600 volt diode is more expensive and generally generates a larger conduction voltage drop than a 300 volt rated diode (which could be used if the voltage ringing did not exist). The increased forward voltage drop induces additional losses in the converter thereby effecting the overall efficiency.
Conventional ways of reducing the ringing voltage in the converter include a "snubber" circuit placed across each rectifier diode which consists of, in one example, a resistor connected in series with a capacitor. The snubber acts as a damping device to reduce the ringing amplitude by dissipating a portion of the ringing energy. While the snubber circuit reduces the reverse voltage across the rectifier diode allowing lower rated devices to be used, it also reduces the overall efficiency of the converter. More specifically, the snubber capacitor causes more current to flow through the rectifier diode when it conducts providing additional energy losses in the converter.
Another technique for reducing the ringing amplitude is to place a saturable reactor in series with the rectifier diode. The saturable reactor is a nonlinear inductor which adopts a lossy characteristic change as the current through it increases to a point where the core material saturates. The saturation characteristic effectively damps the ringing amplitude by dissipating the ringing energy (and reducing the EMI), but it tends to become physically hot and, as a result, is often impractical to use in the converter.
Other damping circuits such as active snubber circuits may also be used in a variety of schemes to reduce the ringing amplitude. Examples of active snubber circuits are illustrated and described in L. H. Mweene, et al., A 1 kW, 500 kHz, front-end converter for a distributed power supply system, Proc. IEEE Applied Power Electronics Conf., March 1989, pp. 423-432; R. Redl, et al., A novel soft-switching full-bridge dc/dc converter: analysis, design considerations and experimental results at 1.5 kW, 100 kHz, IEEE Power Electronics Specialists Conf. Rec., 1990, pp. 162-172; G. Hua, et al., An improved zero-voltage-switched PWM converter using a saturable inductor, IEEE Power Electronics Specialists Conf. Rec., 1991, pp. 189-194; K. Harada, et al., Switched snubber for high frequency switching, IEEE Power Electronics Specialists Conf., 1990, pp. 181-188; V. Vlatkovic, et al., High-voltage, high-power, ZVS, full-bridge PWM converter employing an active snubber, Proc. IEEE Applied Power Electronics Conf., March, 1991, pp. 158-163. The aforementioned references are incorporated herein by reference. The presently available active circuits, however, tend to be complex in nature and generally lack the robustness inherent with the use of passive elements.
Accordingly, what is needed in the art is a robust means to reduce the undesirable ringing amplitude in the converter without significantly effecting the efficiency of the converter.